Communication devices such as terminals are also known as e.g. User Equipments (UEs), mobile terminals, stations (STAs), wireless devices, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a wireless communications network, such as a Wireless Local Area Network (WLAN), or a cellular communications network sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via an access network and possibly one or more core networks, comprised within the wireless communications network.
The above communications devices may further be referred to as mobile telephones, cellular telephones, laptops, tablets or sensors with wireless capability, just to mention some further examples. The communications devices in the present context may be, for example, portable, pocket-storable, hand-held, wall-mounted, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the access network, such as a Radio Access Network (RAN), with another entity, such as an Access Point (AP), another communications device or a server.
Energy efficiency is of paramount importance in many Internet of Things (IoT) applications because IoT devices, such as sensors and other devices, e.g. communications devices, are often battery powered and a long battery life is desired. Therefore, recent and future variants of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard, such as the IEEE 802.11ah standard, must include features to improve the energy efficiency of the IoT devices. On the other hand, extended range is also an important feature for IoT applications, in order to reach places with high penetration losses, for outdoor applications, or simply to help balance the link budget. The IEEE 802.11ah standard includes a new Modulation and Coding Scheme (MCS) called MCS10 that adds a repetition code, e.g. a 2× repetition code, to the MCS0 channel code in order to improve the sensitivity by three (3) dB. There are currently efforts to standardize a Long Range Low Power (LRLP) mode in the IEEE 802.11 standard. Even for LRLP there is an interest in adding a repetition code in order to extend the range. Clearly, increasing the range and decreasing the power consumption are somewhat contradictory goals. Nonetheless both are very important and therefore standards that offer good compromises and flexibility to optimize one or the other according to the use case are sought.
The design of the IEEE 802.11ah air interface was guided by the principle of re-use of hardware and software, in order to make the air interface compatible with earlier versions of the standard. Backwards compatibility accelerates time to market and reduces costs. As an example, the IEEE 802.11ah standard inherited all the MCS from the IEEE 802.11ac standard, which enables the re-use of hardware accelerators that perform Viterbi decoding. Similarly, the current proposal for an enhancement of the IEEE 802.11 standard in the 2.4 GHz bands mentions compatibility with the IEEE 802.11ax standard.
Range extension in the IEEE 802.11ah standard is obtained by simple methods that are to a large extent backward compatible with previous versions of the standard. In addition to using sub 1 GHz carrier frequencies, the IEEE 802.11ah standard employs the following methods.
Firstly, it employs narrow radio frequency channels, which allow a transmitter, e.g. a transmitting node, to increase the power spectral density. The narrowest channel bandwidth in the IEEE 802.11ah standard is 1 MHz.
Secondly, it employs the repetition code, e.g. the 2× repetition code. A new MCS named MCS10 is created starting from the MCS0, e.g. the most robust MCS inherited from the IEEE 802.11ac standard, and adding a 2× repetition code. In theory, the range is increased by 3 dB, at the cost of doubling the length of the packets and doubling the energy consumption.
It should be noted that the design of the MCS10 has the merit of simplicity.
Even though the work in the LRLP area is still in its early stages, a repetition mode has been mentioned as a means to obtain range extension.
Wireless communications systems, such as LTE, have sophisticated retransmission mechanisms. The basic principle is that when a packet is not correctly decoded, a receiver, e.g. a receiving node, sends a non-acknowledgement (NACK) to the transmitting node and a retransmission, possibly with a different channel code, e.g. a different puncturing pattern, is sent from the transmitting node to the receiving node. The receiving node buffers the first packet and performs soft combining upon reception of the second packet. In the simplest retransmission scheme, the same channel code is applied to all the transmissions of the same data, and the receiving node simply buffers and accumulates the soft values for every received packet. This is sometimes referred to as chase combining.
Link adaptation is a term used in wireless communications to denote the matching of modulation, coding and other signal and protocol parameters to the conditions on a radio link, e.g. the radio channel. For IoT devices link adaptation is difficult because the IoT devices are active sporadically and may sleep for very long time periods, so it is difficult to gather reliable up-to-date statistics of the channel conditions. The right choice of the MCS at the AP is very important for energy efficiency. Choosing an MCS that is not robust enough leads to packet re-transmissions, which drains the battery. Similarly, choosing an MCS that is too robust, e.g. choosing the MCS10 when the MCS0 would have been enough, also leads to the unnecessary consumption of significant amounts of power.
The reception of broadcasted management frames such as beacons often consumes a substantial amount of energy. For example, in some cases the battery lifetime of an actuator STA is highly dependent on the beacon length and the beacon frequency, since more than 99% of the total energy consumption is devoted to the reception of beacons, while less than 1% of the energy is actually spent receiving the payload directed to the STA. In addition, broadcasting in the 802.11 systems is not energy efficient because the total time that all the receiving STAs need to be awake, with the RX window open, is determined by the link requirements of the STA with the lowest SNR. The reason is that the MCS used to modulate and code the packets must be robust enough so that the device with the weakest link and lowest SNR is able decode it, and the packet length is directly related to the MCS. For example, in the 802.11ah standard an MCS10 packet is roughly two times longer than an MCS0 packet with an identical payload. Thus, the energy consumed during the reception of an MCS0 packet carrying a given payload is roughly one half of the energy required to receive an MCS10 packet carrying the same payload.
Since energy efficiency is very important in battery operated IoT devices, energy efficient forms of channel coding for broadcasting and power efficient link adaptation methods are sought. For example, in a 802.11ah BSS where some STA's require range extension, the AP may have to encode the beacons using the MCS10, which implies a significant increase in the battery consumption for all those STA's that have moderate to high SNR and that would have been able to receive less robust MCS's, such as the MCS0 or an MCS1.
The article: “PHY Modifications of IEEE 802.11 Systems for transmission at very low SNR” to Langhammer et. al. XP031947907, ISBN 978-1-61284-885-3 discloses physical layer (PHY) modifications of the IEEE 802.11 systems to increase the SNR robustness and communication range by reusing common system components. A spreading code is applied which is equivalent to applying a repetition code. Thus, the disclosed modification may be seen as an alternative to applying a repetition code as in the MCS10 described above. Therefore, the drawback of the proposed modifications is the same as the drawback of applying the repetition code.
WO 2015/061729 A1 discloses a method for generating a physical layer (PHY) data unit for transmission via a communication channel. The PHY data unit conforms to a first communication protocol. Orthogonal frequency division multiplexing (OFDM) symbols for a data field of the PHY data unit are generated according to a range extension coding scheme that corresponds to a range extension mode of the first communication protocol. A preamble of the PHY data unit is generated, the preamble having i) a first portion that indicates a duration of the PHY data unit and ii) a second portion that indicates whether at least some OFDM symbols of the data field are generated according to the range extension coding scheme. The first portion of the preamble is formatted such that the first portion of the preamble is decodable by a receiving device that conforms to a second communication protocol, but does not conform to the first communication protocol, to determine the duration of the PHY data unit based on the first portion of the preamble. The PHY data unit is generated to include the preamble and the data field. WO 2015/061729 A1 provides a solution for range extension. However, the decoding is not energy efficient when receiving devices have good channel conditions. This aspect is critical in IoT applications where the receiving devices are battery powered.